Detection and quantitation of a specific nucleic acid sequence present in a sample is a known diagnostic method with great specificity. This specificity is based on the knowledge of the specific sequence and the generation of probes which are specific and complementary.
Methods for detection and quantitation of specific nucleic acid sequences are illustrated by the following patents: (1) U.S. Pat. No. 5,130,238 is directed toward an improved process for amplifying a specific nucleic acid sequence. The improvement of the amplification process involves the addition of dimethylsulfoxide (DMSO) alone or in combination with bovine serum albumin (BSA); (2) U.S. Pat. No. 4,683,195 is directed toward a process for amplifying and detecting any target nucleic acid sequence contained in a nucleic acid or mixture thereof; (3) U.S. Pat. No. 4,683,202 is directed toward a process for amplifying any desired specific nucleic acid sequence contained in a nucleic acid or mixture thereof; (4) U.S. Pat. No. 4,486,539 is directed toward a method for identifying nucleic acids by a one-step sandwich hybridization test; and (5) WO 91/02814 is directed toward a process for amplifying a specific nucleic acid sequence.
The use of highly specific nucleic acid probes is in some cases the only method which can yield accurate results when the protein is absent, such as is the case for analysis of genetic defects such as cystic fibrosis. It is also valuable in the case of a latent viral infection such as HIV1 or herpes where little or no protein is produced by the infection. The great specificity of the nucleic acid probes also makes them valuable in the diagnosis of infectious agents which are difficult to identify with antibodies due to cross reaction and lack of cross reaction between isotypes of these agents. In addition, the analysis of DNA sequences allows the rapid and effective selection of a probe which will be specific this is not possible with antibody-based reagents.
The greatest difficulty and limitation with applying existing nucleic acid probe technology is the complexity and slow methodology for the detection of specific sequences. With the amplification of systems which require multiple cycles of incubations and multiple washes followed by multiple incubations for detection of nucleic acid.
According to one aspect of the invention, a process for amplifying a specific nucleic acid sequence is used followed by the addition of two oligonucleotide probes—one with a binding species, the capture probe (i.e., biotin or antigen) and one with an electrochemiluminescent label.
The process involves the synthesis of single stranded RNA, single stranded DNA, and double stranded DNA. The single stranded antisense RNA is a first template for a second primer. The single stranded DNA is a second template for a first primer. The double stranded DNA is a third template for the synthesis of a plurality of copies of the first template. A sequence of the first or the second primer is sufficiently complementary to a sequence of the specific nucleic acid sequence and a sequence of the first or the second primer is sufficiently homologous to a sequence of the specific nucleic acid sequence. A second primer binds to the 3′ end of the first RNA template and generates the second DNA template. A 3′ end of the first primer hybridizes to the 3′ end of the second DNA template. The second template is removed from the first template and is used to generate a complementary DNA strand. The resulting duplex DNA serves as a third template for synthesizing a plurality of first templates which in turn repeats the above described cycle. This process of amplification is described in detail by the following publications: Kievits et al., 35 J. Vir Method 273-286 (1991); Bruisten et al., 9 AIDS Res. and Human Retroviruses 259-265 (1993); EP 0 329 822-A2, WO 91/02818, WO 91/02814 (an essentially similar method is also described in WO 88/10315). On completion of the incubation, as described above, samples from the said amplification are taken and a mixture of complementary probes and beads coated in a binding species complementary to one of the probes in hybridization buffer is added followed by incubation at a predetermined temperature to allow the hybridization of the probes to the said first template and the binding of one of the said probes to the beads via a binding interaction, i.e., antibody-antigen or biotin-streptavidin. On completion of the said incubation, a complex is formed which comprises the said first template generated from the amplification reaction as above hybridized to two said differing probe, one containing an electrochemiluminescent label and the other a binding species. This complex is further complexed to said coated bead which forms its complementary binding pair with the probe binding species. The resulting complex contains the amplified first template, the probe with its electrochemiluminescent label, the capture probe with its binding species, and the bead with its coating of binding species (see FIG. 1) all complexed via the specific interactions of each component. It will also be understood that the DNA sequences generated during the NASBA cycling will be targets for hybridization and detection.
In another embodiment of the claimed invention, the interaction between the probe and the bead can be made prior to the hybridization step by formation of a covalent bond to said bead or via a binding species (where said binding species is coated either by covalent or non-covalent methods) interaction or indirectly via a covalent bond to a species which can non-covalently coat said bead. An example of this indirect covalent coating could take the form of the probe being coupled to a carrier such as protein followed by coating via non-covalent methods to the bead surface.
In another embodiment, samples of the amplification would be mixed with a probe labeled with an ECL species and a bead which is coated with a binding species specific for the hybrid formed between said probe to the said amplified first template. For example, such a hybrid of DNA and RNA may be capture using a specific antibody. An example would be the use of a anti DNA:RNA antibody (Miles Inc. U.S. Pat. No. 4,833,084). Other antibodies which recognize such mixed hybrid molecules would also prove valuable such as those antibodies raised to hybrids of RNA or DNA to phosphonate, phosphorothioate, alkyl, or aryl phosphonate based nucleic acid sequences (Murakami et al., 24 Biochemistry 4041-4046 (1985), also available from Glenn Research, Sterling, Va.). These methods are based on the formation of a new molecular species on hybridization which is a binding species for an antibody and raising antibodies or other binding species to these molecular species.
In yet another embodiment, the assay method may also be used to quantitate the amount of nucleic acid in the starting material. This is achieved by the addition to the samples of specific ‘spike’ samples which are amplified during the reaction. The determination of the spike signal and sample signal allows a ratio to be calculated, which based on the original spike level, allows the determination of the sample level. This is most accurately determined by the use of multiple spikes which range in amount over the range of the potential sample amounts and allow the construction of a standard curve to give a reading at a ratio of 1:1 between target and sample. Methods based on this are well understood—Van Gemen et al., 43 J. Vir. Methods 177-187 (1993); Siebert and Larrick, 14 Biotechniques 244-249 (1993); Piatak et al., 14 Biotechniques 70-80 (1993). This method for quantitation is improved by the use of a rapid and accurate method for detection and quantitation provided by the use of the ECL labels and methods with the NASBA amplification.
The invention further provides a process for the detection of a specific nucleic acid sequence, comprising the steps of:    (a) Providing a single reaction medium containing reagents comprising            (i) a first oligonucleotide primer,        (ii) a second oligonucleotide primer comprising an antisense sequence of a promoter,        (iii) a DNA-directed RNA polymerase that recognizes said promoter,        (iv) an RNA-directed DNA polymerase,        (v) a DNA-directed DNA polymerase,        (vi) a ribonuclease that hydrolyses RNA of an RNA-DNA hybrid without hydrolyzing single or double-stranded RNA or DNA,            (b) Providing in said reaction medium RNA comprising an RNA first-template which comprises said specific nucleic acid sequence or a sequence complementary to said specific nucleic acid sequence, under conditions such that a cycle ensues wherein            (i) said first oligonucleotide primer hybridizes to said RNA first template,        (ii) said RNA-directed DNA polymerase uses said RNA first template to synthesize a DNA second template by extension of said first oligonucleotide primer and thereby forms an RNA-DNA hybrid intermediate,        (iii) said ribonuclease hydrolyses RNA which comprises said RNA-DNA hybrid intermediate,        (iv) said second oligonucleotide primer hybridizes to said DNA second template,        (v) said DNA-directed DNA polymerase uses said second oligonucleotide primer as template to synthesize said promoter by extension of said DNA second template; and        (vi) said DNA-directed RNA polymerase recognizes said promoter and transcribes said DNA second template, thereby providing copies of said RNA first template; and thereafter            (c) Maintaining said conditions for a time sufficient to achieve a desired amplification of said specific nucleic acid sequence, followed by the addition of:            (i) at least one probe sequence complementary to said RNA first template labeled with an electrochemiluminescent species,        (ii) at least one second capture probe sequence complementary to said RNA first template labeled with a binding species,        (iii) a bead coated with a complementary binding species to said second probe sequence; and thereafter            (d) Providing conditions of temperature and buffer to allow the hybridization of the probes to the said RNA first template and the binding of said binding species on said second capture probe with the complementary binding species on said bead to from a bead bound complex; and then    (e) Detecting said bead bound complex using said electrochemiluminescent species.
The invention further provides a process for the detection of amplified products comprising the steps of:    (a) amplifying a sample nucleic acid under conditions to generate amplified product;    (b) mixing said amplified product with two binding species comprising            (i) an ECL labeled binding species which interacts with a trimolecular complex with the amplified nucleic acid and bivalent binding species;        (ii) a bivalent binding species which interacts with a trimolecular complex with the amplified nucleic acid and ECL labeled binding species;         to form a binding complex reaction;            (c) incubating said binding complex reaction under conditions which allow the formation of a trimolecular complex of amplified product, ECL labeled binding species, and bivalent binding species;    (d) capturing said trimolecular complex via the bivalent binding species' remaining binding site to a solid phase; and    (e) quantitating ECL label captured on the solid phase.Definitions
In order to more clearly understand the invention, certain terms are defined as follows:
“Amplified product” means nucleic acid sequences generated by copying sample nucleic acid sequences multiple times using an enzymatic reaction.
“Annealing” refers to hybridization between complementary single chain nucleic acids when the temperature of a solution comprising the single chain nucleic acids is lowered below the melting or denaturing temperature.
“Binding species” means any species known to bind with another molecular species and are normally defined as a pair of species but may be formed from higher complexes, i.e., 3 or 4 which bind, i.e., antibody:antigen or oligonucleotide:antibody or oligonucleotide:antigen or DNA:DNA or DNA:RNA or RNA:RNA or DNA:RNA:DNA or Biotin-DNA:DNA-ECL labeled or receptor:ligand or DNA binding protein such as restriction enzymes, lac repressor.
The “complement” to a first nucleotide sequence is well known to be a second sequence comprising those bases which will pair by Watson-Crick hybridization with the first sequence. Thus, the complement to the deoxyribonucleic acid (DNA) sequence 5′ATGC 3′ is well known to be 5′-GCAT 3′. For duplex, or double stranded DNA, each of the two strands are described as complementary to the other or as a complementary pair. The terms complement and anticomplement may also be used. With reference to the identification of the strand of duplex DNA from which transcription to RNA proceeds, the transcription strand is generally described as plus and its complement as minus (“+” and “−”), or the transcription strand may be described as the sense strand, and its complement as antisense. Two strands each hybridized to the other having all base pairs complementary, are 100% complementary to each other. Two strands, each hybridized to the other, have 5% of bases non-complementary, are 95% complementary (or the two strands have 95% complementarity). In addition, it will also be understood that nucleic acid sequences can form triple helix structures based on a specific interaction of three strands which would be considered to complementary in a specific way to each other within this triple stranded hybrid.
The terms “detection” and quantitation” are referred to as “measurement”, it being understood that quantitation may require preparation of reference compositions and calibrations.
“Electrochemiluminescent (ECL) species” means any compound known to electrochemiluminescense;
“Electrochemiluminescent (ECL) labels” are those which become luminescent species when acted on electrochemically. Electrochemiluminescent techniques are an improvement on chemiluminescent techniques. They provide a sensitive and precise measurement of the presence and concentration of an analyte of interest. In such techniques, the sample is exposed to a voltammetric working electrode in order to trigger luminescence. The light produced by the label is measured and indicates the presence or quantity of the analyte. Such ECL techniques are described in PCT published application by Bard et al. PCT Appl. No. US 85/02153, entitled “Luminescent Metal Chelate Labels and Means for Detection” and Massey et al. PCT Appl. No. US 87/00987, entitled “Electrochemiluminescent Assays”; PCT Appl. No. US 88/03947, Publication No. WO 89/04302 “Electrochemiluminescent Moieties and Methods for Their Use”; Hall et al., “Method and Apparatus for Conducting Electrochemiluminescent Measurements”, U.S. application Ser. No. 744,890 filed Aug. 14, 1991; and Zoski and Woodward. “Apparatus for Conducting Measurements of Electrochemiluminescent Phenomena”, PCT US 89.04854 corresponding to pending EPO Appl. No 89/912913.4, Publication No. 0441880.
Examples of ECL tags are tag NHS (N-hydroxy-succinimide) and tag phosphoramidite. The tag-NHS ester is useful for labeling substances containing free amino groups capable of reaction with the NHS ester to form an amide bond. (See, for example, WO 86/02734.) The tag phosphoramidite (Gudibande et al. U.S. application Ser. No. 805,537 f entitled “Improved Electrochemiluminescent Label for DNA Probe Assay” which is hereby incorporated herein by reference) is useful for labeling substances containing free amino, sulphydryl, or hydroxyl groups forming phosphor-linkages especially phosphodiester linkages.
An “ECL assay buffer” is a general diluent which contains tripropylamine that is necessary for the electrochemical reaction on the electrode in an ECL analyzer.
An “ECL diluent” is a diluent reagent used in diluting solutions containing labile biomolecules for storage purposes.
“ECL apparatus” is any apparatus for performing electrochemiluminescence based assays. Such ECL apparatus are described in PCT Appl. No. US 85/02153 by Bard et al. entitled “Luminescent Metal Chelate Labels and Means for Detection” and in PCT Appl. No. US 87/00987 by Massey et al. entitled “Electrochemiluminescent Assays”; PCT Appl. No. US 88/03947, Publication No. WO 89/04302 “Electrochemiluminescent Moieties and Methods for Their Use”; Hall et al. “Method and Apparatus for Conducting Electrochemiluminescent Measurements”, U.S. application Ser. No. 744,890; and Zoski, G., and S. Woodward. “Apparatus for Conducting Measurements of Electrochemiluminescent Phenomena”, PCT US 89.04854 corresponding to pending EPO Appl. No. 89/912913.4, Publication No. 0441880.
“Homology” between polynucleotide sequences refers to the degree of sequence similarity between the respective sequences. Two strands which are identical in sequence have 100% sequence homology. Two strands which differ by 5% of sequences have 95% sequence homology. The greater the degree of homology between two strands A and B, the greater the complementarity between A and the complement of B.
“Hybridization” describes the formation of double stranded or duplex nucleic acid from complementary single stranded nucleic acids. Hybridization may take place between sufficiently complementary single stranded DNA and/or RNA to form: DNA:DNA, DNA:RNA, or RNA:RNA or DNA:RNA:DNA or Biotin-DNA:RNA:DNA-ECL label. This may also include sequences of nucleotides which are linked using modified natural chemistries such as phosphonate, phosphorothioate, alkyl or aryl phosphonate based nucleic acid sequences (Murakami et al., Biochemistry 24 (1985):4041-4046, also Glenn Research, Sterling, Va.).
The term “label” or “labeled” when applied to a nucleic acid means that the nucleic acid in question is linked to a moiety which is detectable by its properties which may include: ECL and luminescence, catalysis of an identifying chemical substrate, radioactivity, or specific binding properties. Thus, the term “label” includes ligand moieties unless specifically stated otherwise.
It is also well know to the art that the term “nucleic acid” refers to a polynucleotide of any length, including DNA or RNA chromosomes or fragments thereof with or without modified bases as described herein.
A “nucleotide” is at least one of the following bases: adenine, cytosine, guanine, thymine or uracil, plus a sugar (deoxyribose for DNA, ribose for RNA), plus a phosphate. In order to provide monomers for the DNA polymerization reaction, typically all four of the deoxynucleotide triphosphates are required. A nucleotide, as defined herein, may also include modified bases such as 5-methyl-dCTP and 7-deaza-dGTP used to improve the action of polymerase on templates. The term nucleotide as used herein also includes bases linked to biotin and digoxigenin (Digoxigenin-11-UTP from Boehringer Mannheim, Indianapolis, Ind.) and biotin-21-UTP and amino-7-dUTP (Clontech, Palo Alto, Calif.) and ECL labeled nucleotide (see FIGS. 6 and 7) which may be incorporated directly into a primer or into a primer extension product during amplification, to provide for selective binding of amplified sequences.
An “oligonucleotide” is a sequence formed of at least two nucleotides.
A “polynucleotide” is a long oligonucleotide and may be either RNA and DNA.
While the term oligonucleotide is generally used in the art to denote smaller nucleic acid chains, and “polynucleotide” is generally used in the art to denote larger nucleic acid chains including DNA or RNA chromosomes or fragments thereof, the use of one or the other term herein is not a limitation or description of size unless expressly stated to be.
A “primer” is a relatively short segment of oligonucleotide which is complementary to a portion of the sequence of interest (the sequence of interest can be a subfragment within a larger nucleic acid sequence). A primer represents a 5′ terminus of the resulting extension product. A primer which is complementary at its 3′ terminus to the sequence of interest on the template strand enables this 3′ terminus to be acted on by a polymerase on hybridization to the template. It is well known that modifications to the 3′ end will affect the ability of an oligonucleotide to function as primer. An example is the incorporation of a dideoxynucleotide as in DNA sequencing thus preventing the action of DNA polymerases. It is well known that the length of the primer will depend upon the particular application, but that 20-30 base pairs is a common size. As is well known, a primer need not be a perfect complement for successful hybridization to take place. If the primer is an imperfect complement, an extension product will result which incorporates the primer sequence, and during a later cycle, the complement to the primer sequence will be incorporated into the template sequence. Thus, it is well known that a properly selected primer having a sequence altered from that of the complement of the template may be used to provide in vitro mutagenesis. The primer may incorporate any art known nucleic acid bases, including any art known modified or labeled bases as defined above so that the primer extension product will incorporate these features to permit separation and detection of the primer extension product. A tag or marker advantageously linked to a primer may include an ECL fluorescent or luminescent tag, an isotopic (e.g., radioisotope or magnetic resonance) label, a dye marker, an enzyme marker, an antigenic determinant detectable by an antibody, or a binding moiety such as biotin enabling yet another indicator moiety such as a streptavidin coated bead to specifically attach to the primer or any nucleic acid sequence incorporating that primer. When the labeled or tagged amplification product is formed, that amplification product may be detected by the characteristic properties of the tag or label.
The term “primer extension product” describes the primer sequence together with the complement to the template produced during extension of the primer.
A “probe” is a single or double stranded nucleic acid which has a sequence complementary to a target nucleic acid sequence of interest and which has some additional feature enabling the detection of the probe—target duplex. One skilled in the art will understand that if the probe and/or the target is double stranded, the double stranded nucleic acid must undergo strand separation before hybridization can take place. It is possible, if a triple strand formation is used, then the double stranded target will not need to be rendered single stranded prior to hybridization.
A probe is rendered detectable by an attached tag or marker. A tag or marker linked to a probe may include a fluorescent, ECL or luminescent tag, an isotopic (e.g., radioisotope or magnetic resonance) label, a dye marker, an enzyme marker, an antigenic determinant detectable by an antibody, or a binding moiety such as biotin enabling yet another indicator moiety such as a streptavidin coated bead to specifically attach to the probe. When the labeled or tagged probe—target duplex is formed, that duplex may be detected by the characteristic properties of the tag or label. Alternatively, as described for the ECL assays in the following examples, the probe with its binding moiety allows the capture of labeled target, via hybridization and duplex formation, allowing detection by a label or other art known means.
“Sample” means a mixture containing nucleic acids.
A “sequence” (e.g., sequence, genetic sequence, polynucleotide sequence, nucleic acid sequence) refers to the actual enumerated bases (e.g., ribose or deoxyribose) present in a polynucleotide strand (e.g., reading from the 5′ and 3′ direction) and the relative position of these bases with respect to each other.
The term “single primer” means a single, unpaired, specific or selected primer designed to selectively hybridize with a target nucleic acid sequence of interest.
“Specific nucleic acid sequence” means a single stranded or double stranded nucleic acid which one could use as a probe or amplify.
A “specific or selected” nucleotide sequence refers to a particular sequence distinguishable (i.e., by hybridization-analysis) from other difference sequences (e.g., the specific nucleotide sequence 5′-ATGCCC-3′ is not the same sequence as 5′-AAGCCC-3′).
A specific or selected primer is one which is designed to hybridize with a particular template sequence to achieve the desired result by making the primer complementary or approximately complementary to the 3′ terminal of the template sequence. The specific primer will selectively achieve the desired result even if the target template sequence is present in a mixture of many other nucleic acid sequences.
The specific or selected primer is distinguished from a “universal primer” which will indiscriminately anneal to any DNA sequence to which a complementary (to the primer) adaptor terminal sequence has been attached. With a universal primer, care must be taken to isolate the nucleic acid of interest, or otherwise direct the ligation procedure only to the desired DNA sequence of interest, to avoid randomly attaching the adaptor to all nucleic acid sequences present.
A “strand” is a single nucleic acid sequence. Thus, a duplex or double stranded chromosome, chromosome fragment or other nucleic acid sequence may be separated into complementary single strands.
“Strand separation” refers to the conversion of a double stranded or duplex nucleic acid to two complementary single stranded polynucleotide. The separation process may employ well known techniques including: enzyme mediated separation (e.g., by the enzyme helicase, physical-chemical separation (pH, ionic concentration and the like), and thermal separation also known as thermal denaturing. Thermal denaturing (also referred to as “melting”) is the separation of a double stranded polynucleotide (fully or partially duplex) into at least two single strands of polynucleotide by raising the temperature of the solution holding that polynucleotide.
“Sufficiently complementary” means that two nucleic acids are capable of specific interaction which allows either a primer dependent and template directed synthesis of DNA or a probe to bind to nucleic acid sequence.
A “template” is any sequences of nucleic acid upon which a complementary copy is synthesized. This may, in general, be DNA to DNA replication, DNA to RNA transcription, or RNA to DNA reverse transcription. A DNA template provides the sequence information for extension of the complementary primer by the DNA polymerase reaction. An RNA template may provide the sequence information for extension of a complementary DNA primer by an analogous reaction catalyzed by the enzyme reverse transcriptase As is well known to the art, the template may be found in a single or double stranded form. If the template enters the amplification process in the double stranded form, the template strand will not hybridize to its complementary primer until it is denatured by the first thermal denaturing cycle. If the template enters the amplification process already in the single stranded form, the primer will hybridize (described as annealing when thermal cycling is utilized) with its complementary template before the first thermal denaturing step.